Determining the distance to measurement points forms the foundation for a variety of measuring tasks and for corresponding measuring devices. In particular, optical distance measurement is used, for example, for measuring devices in surveying (geodesy) or in industrial workpiece testing and/or measurement. In this case, for example, coordinates of points on a workpiece to be monitored can be determined. The advantages of these methods are in particular a broad area of application as a result of the comparatively long measurement range and the comparatively high measurement accuracy, which can be provided, for example, by interferometric distance measurement.
The need exists in many technical and/or industrial areas of application for measuring surfaces of objects and therefore also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the measuring and checking of surfaces of produced workpieces has great significance, in particular also for purposes of quality control.
Coordinate measuring machines are typically used for these applications, which enable a precise measurement of the geometry of an object surface, typically with micrometer accuracy. Objects to be measured can be, for example, engine blocks, transmissions, and tools. Known coordinate measuring machines measure the surface, for example, in that a mechanical contact is produced and the surface is scanned. Examples of this are gantry-type measuring machines, as are described, for example, in DE 43 25 337 or DE 43 25 347. Another system is based on the use of an articulated arm, the measuring sensor of which, which is arranged at the end of the multipart arm, can be moved along a surface. Articulated arms of the type in question are described, for example, in U.S. Pat. No. 5,402,582 or EP 1 474 650. U.S. Pat. No. 5,822,877 describes a multiprobe system for use on a CMM, consisting of several individual probes mounted side by side on a support structure.
In addition, the use of optical measuring sensors in coordinate measuring machines has become routine. The optical sensors used for this purpose are based on irradiation of, for example, laser light onto an object surface for interferometric measurements (EP 2 037 214). Methods based on white light interferometry (DE 10 2005 061 464) and chromatic-confocal methods (FR 273 8343) are also known. The measuring sensors may be exchanged flexibly and automatically via optomechanical coupling elements on the coordinate measuring machine (EP 02356401).
Optical sensors and/or optical measuring methods for a coordinate measuring machine are linked to an array of advantages: the measurement is performed in a contactless manner, and the optical sensor can be guided more rapidly than a tactile sensor over an object surface, with smaller physical dimensions of the “measuring tip”, whereby a higher lateral resolution of the measurement is enabled.
However, the mentioned optical measuring methods share the disadvantage of distance measurement with reduced accuracy in the event of unfavorable environmental influences, for example, vibrations on the measuring device. Additional limitations may arise on surfaces which are difficult to measure, for example, which cause strong scattering of the measuring radiation or have an unfavorable roughness with respect to the selected radiation properties. Furthermore, the problem exists that frequent exchange of the optical sensor has to be performed in the case of complex workpiece geometries, to provide the matching sensor for a respective surface section. For example, a pivotable sensor is suitable for scanning a curved surface. However, it could be oriented in each case so that the measuring radiation is incident nearly orthogonally on the object surface to achieve optimum measuring conditions.
However, such a sensor would be entirely unsuitable for measuring a borehole, because the measuring radiation thus could not be oriented in the direction of the borehole circumference, but rather solely axially in the direction of the borehole depth. The measuring sensor thus has to be exchanged for such a measurement. This exchange procedure generally requires a comparatively large amount of processing time, whereby the overall time consumed by a measuring procedure increases significantly.
On the other hand, using a continuous probe head allows to be normal to surface, however this alters the accuracy of the system.
A similar problem results in the case that, for example, one section of an object is to be captured with greater accuracy than another section of the object. Different optical sensors would also typically be used for this purpose—a low-resolution sensor to do a rapid scanning, and a second high-resolution sensor, to measure with increased accuracy.
Such a requirement for an exchange of the measuring sensor, to adapt, for example, to the object geometry, can reduce the efficiency of a measuring process.